I know i have the wrong combo. I am trying to fix it. Any ideas were to start. I need to have my 427 8-71 5lbs of boost motor shift around 6000rpms. I went to the track this weekend and my bbc reved up way to quickly to my 6400 rev limiter even in 3rd gear on part throtal. Heres my combo
31/14/15 tire
4.88 gears
Solid roller cam 3200-7200 range 112 lobe .714 lift
4500 converter th400
3400lbs car

Nitrous or supercharged engines affect converter selection. An engine with super charger produces more torque than it would if it was normally aspirated ( your running very low boost for a drag car) That means a blown engine needs a converter with a lower stall speed range. Otherwise, the converter will stall too high, causing it to slip and the engine will rev too quick and eventually the converter will self-destruct due to the extra heat generated. Your converter may also be ballooning!

Whats the overlap on the cam, Or give he cam specs? Whats your compression?remember, with a supercharger you aren't relying on the natural vacuum of the engine to pull in air, it's already there and in more abundance than would normally be without the Super charger. that means it's going to force air out through the exhaust valve during overlap. excessive overlap isn't something you want on a boosted engine, supercharger

but aside from the overlap, it's not necessary to have your valves open up for so long either. those kind of cams make a lot of power really high in the rpm range, where superchargers start to lose efficiency. not to mention it causes a lot more stress on the springs since they have boost pressure to contend with also.

Jester

Last edited by painted jester; 12-09-2012 at 11:40 PM.

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Thxs jester for the info. Heres the cam info. Im only running 5lbs cause i want to walk into this blower set up carefully and im running a btm also. I was running 12.1 sec. normally aspirated and even with 5lbs of boost i can feel a big differ in torque at half throtal.
Dart iron eagles 345cc, 2.25 valve
Doug herbert cam .715/.710. @50 260/270. 305/310
8.7 compression dish forged pistons.
2/850 holley dp with all powervalves plugged with 85 jets all the way around. Yes it runs rich as expected with no power valves.
A balloon plate was installed on converter.
Can you give me a ballpark idea of what i need to have a 6k shift point with the same cam.
Stall? Gears? Or no choice but scrap the cam and find one in the 6000 range.?

Thxs jester for the info. Heres the cam info. Im only running 5lbs cause i want to walk into this blower set up carefully and im running a btm also. I was running 12.1 sec. normally aspirated and even with 5lbs of boost i can feel a big differ in torque at half throtal.
Dart iron eagles 345cc, 2.25 valve
Doug herbert cam .715/.710. @50 260/270. 305/310
8.7 compression dish forged pistons.
2/850 holley dp with all powervalves plugged with 85 jets all the way around. Yes it runs rich as expected with no power valves.
A balloon plate was installed on converter.
Can you give me a ballpark idea of what i need to have a 6k shift point with the same cam.
Stall? Gears? Or no choice but scrap the cam and find one in the 6000 range.?

I'm not sure I understand your dilemma. If you set a shift light to 5800 (or whatever you need to set it to, to not exceed 6000 rpm), you should be able to upshift manually and not exceed 6000 rpm. The thing is, by short shifting like that, you're not going to get all the cam has in it so the ET might not be anything special, until or unless you run it out to the cam's max HP rpm before you shift.

The 4.88 rear gears are magnifying the fast revving engine. If you were to change the gears to 4.11, say, the rpm will be rising a little slower, but the potential best ET might be hurt if you have tall slicks. But generally speaking, you don't usually use that much gear for a street/strip car w/a blown BBC engine

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My issue is that as soon as i try to give it gas it jumps quickly to 6000 and i shift and rpms stay high at 6000 so i shift to third and i hit my rev limiter at 6400 with only part throtal at half track in 1/4 mile. I want to slow down the rise of rpm so i can have a decent. 2nd and 3rd. Hope this makes sence. Is 4.10 a good ratio. How do you do the math to select ratio?
Only sees the track.

The 4.88 rear gears are magnifying the fast revving engine. If you were to change the gears to 4.11, say, the rpm will be rising a little slower, but the potential best ET might be hurt if you have tall slicks. But generally speaking, you don't usually use that much gear for a street/strip car w/a blown BBC engine

My issue is that as soon as i try to give it gas it jumps quickly to 6000 and i shift and rpms stay high at 6000 so i shift to third and i hit my rev limiter at 6400 with only part throtal at half track in 1/4 mile. I want to slow down the rise of rpm so i can have a decent. 2nd and 3rd. Hope this makes sence. Is 4.10 a good ratio. How do you do the math to select ratio?
Only sees the track.

I think I understand you now- it was my problem, not yours.

Jester has the right idea, I believe. I'm thinking the torque of that bad boy is driving right through the converter, causing it to stall way too high (you can think of this as the same type thing as a slipping manual clutch). Or the torque converter itself is faulty. This amount of slippage just tortures the ATF and can break the fluid down to the point of a transmission failure from the heat and loss of lubricity of the ATF. For that reason I'd recommend you not race this until you get the problem fixed.

Now, I am assuming the tranny isn't what's doing the slipping and that the fluid is at the correct level and the filter isn't clogged.

Check the color and smell of the ATF. By now the fluid may have a bad smell and be darker than normal. If this is what you see/smell, the ATF is no good and has to be changed along w/the filter. If you have to change the fluid and filter, be sure to look at what's in the bottom of the tranny pan and stuck to the magnet (if there's a magnet in the bottom of the pan). If you see pieces of friction material and/or metal debris, you might have more problems than a too-loose torque converter.

Hopefully the trans is still OK. If it is, I believe the cure is going to be a tighter blower torque converter and maybe higher rear gears, like the 4.11 you mentioned. But start w/the tranny/converter first.

For the torque converter, I wouldn't try to cut corners. Instead, get the advice of a good converter builder like Coan, Hughes, or Neil Chance.

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A supercharged engine has so much torque off idle all the way through your rpm range that you lose torque with a high stall converter because of slippage! its not like a naturally aspirated engine that's torque comes in at say 2800 rpm and you have to rev to 2800 rpm with a trans break and a 3000 stall converter to use the torque efficiently to get a good launch !

A 3000 stall converter is a judgement measurement of the manufacturers on usually break stall at a predetermined torque, but flash stall of a converter is a different story!!!! A 2000 stall converter that might flash stall at 2500 rpm (1to1) with 350 pounds torque instantly Flashed (applied to it , a drag strip launch)! But with more torque the same 2000 converter Might flash stall at 3500 rpm with 450 Pounds of torque applied instantly with all other conditions remaining the same!!!

So when your supercharged big block applies all its torque in a flash() your 4500 convertors impeller and turbine rotation cant catch up to matching RPM (or reach 1to1) so your engine rpm peaks out really fast (like your "clutch is slipping" as Cobalt said) and when you speed shift to the next gear the converter is still slipping and your torque cant be used its power is eaten up by the converter (like a parasite) and the engines revs fast again with no gain of power!

I hope that helps you understand what Cobalt and I are saying! He gave you really good info! Hes also right about the gears you don't need the big gears to get you a good launch with a good supercharger set up and the 850s sound about right ( its better to run a little rich) I ran 33 lbs boost with 2 950s on a small block 355 with 5 to 1 compression ,a 4 speed and 456 gears, 10.3 and 133 mph at the top of the quarter I was in my peek rpm range in the top of 4th gear!! If I was in the bottom of 4th I would have gone to 4.88 gears or run shorter slicks

Your cam with the big lift and overlap can be robbing boost and your cam advanced or retarded can rob more? I ran "Blower Drive Service" racing cams designed for their blowers and configurations! And their tech. people were good! But many cam company's grind good blower cams!

If you need formulas to figure out top rpm at the 1/4 mile,tire size, gears,speed Etc. just ask on here Cobalt , myself, and others on here can help you!

I reread this and hope it makes sense to you

If I made an error in it someone will correct me LOL

Jester

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Wow i didnt think it would be this much drama with 5lbs of boost. Do you guys think i can keep the cam? I really like it and it is a niitrous cam profile from herbert? It sounds like the converter is toast. Will look into hughes there around the corner. Do you think i can stay in the 6000 shift point range with change of gear and converter. I wonder what size convter they will recommend.thxs guys i think i just made some new friends happy holidays.

Wow i didnt think it would be this much drama with 5lbs of boost. Do you guys think i can keep the cam? I really like it and it is a niitrous cam profile from herbert? It sounds like the converter is toast. Will look into hughes there around the corner. Do you think i can stay in the 6000 shift point range with change of gear and converter. I wonder what size convter they will recommend.thxs guys i think i just made some new friends happy holidays.

Providing it's the converter causing the slippage, a tighter converter spec'ed to your engine will make all the difference and will let you shift at 6000 rpm if you so choose. But like I said, the cam's powerband is higher than 6000 rpm, so expect the et to not be as good as it could be. Still ought to be stout, though.

If you plan to limit the engine to 6000 rpm, there are better cams- it's too big to limit it like that.

While watching my in board video of the tach and boost gauges of my last run. Tell me what you think if this is an idicator of bad converter or tranny.? When i shifted to 2nd at 6400 i noticed my tach jumped up to 6700 then dropped to 6400. I would expect the tach to drop to around 5900 not go up. And doing the gear ratio conversion for my tire it turns out to be a 5.19 holy cow.

A GOOD converter company thats familiar with supercharged engines will be your best call/investment. You need to research them out. I trust PTC implicitly with my BBC and my sbc normally aspirated motors. I know they also do sprayed and turbo motors. I DO NOT know if they are competent with S/C, but would assume the increased torque of N2o will be close to your 5#. A quick call willl give you the answers you need. Lane is very good there. If he cant do it, he will tell you.

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